1. Field of the Invention
The present invention relates to an image pickup apparatus, and more particularly to an image pickup apparatus capable of properly correcting a white balance and luminance level of an image illuminated by a light source having flickers.
2. Related Background Art
As an image pickup apparatus, there are known a video camera, a still video camera, and the like. An automatic tracing method of tracing a change of light from a light source is known as a method of adjusting a white balance. A fluorescent lamp used as a light source generates flickers. Therefore, an image taken under illumination by a light source of a fluorescent lamp becomes sometimes bluish or reddish. A proper white balance cannot be obtained with the automatic tracing method if the light amount incident to an image pickup device becomes different from the light amount incident to a white balance adjusting sensor, because of the influence of flickers. Such a case will be described more in detail with reference to FIG. 1.
FIG. 1 (i) shows a change of color of a fluorescent lamp caused by flickers. Red is above the line of a mean value, and blue is under the line. FIG. 1 (ii) and (iii) show the accumulation times of a charge coupled device serving as an image pickup device. FIG. 1 (ii) shows the accumulation time at a shutter speed 1/60 sec, and FIG. 1 (iii) shows the accumulation time at a shutter speed 1/1000 sec.
As seen from FIG. 1, since the accumulation time of a CCD is sufficiently long for the case of FIG. 1 (ii), an output of the CCD, i.e., an output mean value during the accumulation time takes a value substantially near the mean value of the light shown in FIG. 1 (i). However, the accumulation time for the case of FIG. 1 (iii) is short so that in some cases the output mean value of the CCD takes a value considerably different from the mean value of the light source. A colorimetry sensor used for adjusting a white balance generates colorimetry data at a very short instant of time as shown in FIG. 1 (iv). Therefore, the colorimetry data sometimes takes a value considerably different from the mean value of the light source depending upon the data read time. Therefore, the white balance adjustment becomes sometimes incorrect in both the cases of (ii) and (iii).
To avoid such incorrect adjustment, it has been proposed to use mean value data by integrating data taken by a colorimetry sensor for a predetermined time as shown in FIG. 1(v). In FIG. 1, if the colorimetry data for 10 ms is averaged and used as an output of the colorimetry sensor, the influence of flickers can be eliminated. In this case, if the low shutter speed shown in FIG. 1 (ii) is used, both the output of the CCD and the output mean value of the colorimetry sensor become near the mean value of the light source, resulting in a small error of the white balance.
However, if the high shutter speed shown in FIG. 1 (iii) is used, the output value of the CCD takes sometimes a value considerably different from the mean value of the light source.
In order to solve the above problem, as disclosed in Japanese Laid-open Patent Application No. 61-238191, it has been proposed to make the accumulation time of an image pickup device coincide with the output integrating (averaging) time of a colorimetry sensor.
With this method, however, making the input accumulation time of an image pickup device coincide with the integrating and averaging time of a colorimetry output of a colorimetry sensor results in the following disadvantages.
In an image pickup apparatus, before an output of an image pickup device starts being read after the accumulation of the image pickup device, a control voltage for adjusting the white balance should be established, the control voltage being derived from a colorimetry mean value obtained from a colorimetry sensor. In this case, the rise time of the output control voltage becomes significant. In some cases, it becomes impossible to use a noise eliminating low-pass filter in a control voltage output circuit. Furthermore, it takes a certain time for a microcomputer to execute a digital calculation so that it cannot be used for another processing during such time. These problems hinder an effective white balance adjustment.